Method and apparatus for determining the composition of a liquefied gaseous mixture



June 14, 1966 l. B. MAYFIELD METHOD AND APPARATUS FOR DETERMINING THE COMPOSITION OF A LIQUEFIED GASEOUS MIXTURE 3 Sheets-Sheet 1 Filed Jan. 15, 1965 .@rLv

June 14, 1966 1. B. MAYFIELD 3,255,594

METHOD AND APPARATUS FOR DETERMINING THE COMPOSITION OF A LIQUEFIED GASEOUS MIXTURE Flled Jan l5, 1963 3 Sheets-Sheet a A: DI 1 June 14, 1966 B. MAYFIELD 3,255,594

METHOD AND APPARATUS Fon DETERMINING THE COMPOSITION oF A LIQUEFIED GAsEous MIXTURE Filed Jan. l5, 1965 3 Sheets-Sheet 3 fraai- United States Patent O METHOD AND APPARATUS FOR DETERMINING THE COMPOSITION OF A LIQUEFIED GASEOUS MIXTURE Ivan B. Mayfield, Lawrenceville, Ill., assignor to Texaco Inc., New York, N.Y., a corporation of Delaware Filed Jan. 15, 1963, Ser. No. 251,672 4 Claims. (Cl. 62-21) This invention is directed to a method and apparatus for the determination of the boiling temperature of a component and the -composition of a vaporizable liquid mixture. More particularly, it relates to a method and apparatus for determining a criterion of the composition of a volatile mixture comprising a major portion of a normally gaseous constituent and a minor portion of a less volatile constituent wherein said liquid mixture is introduced into a first vaporization zone at a constant rate vaporizing a portion of said liquid and concomitantly cooling remaining liquid, said cooled remaining liquid is passed to a second vaporization zone evaporating all but a minor residue of said remaining liquid, said minor residue is continuously withdrawn, and the temperature of said minor residue withdrawn from second vaporization zone is measured.

In the manufacture and utilization of volatile liquid products, it is frequently desirable to determine the presence and relative amount of minor constituents higher boiling than the main body of liquid. For example, in the separation of materials by fractional distillation, complete separation of absolutely pure product is impossible and some higher boiling material appears in the distillate fraction and some of the desired distillate component remains in the distilland and appears in the distillation bottoms. Ecient fractional distillation requires that a compromise be made between the purity of the desired distillate fraction and the complete stripping of the desired distillate components from the higher boilingconstituents. An object of this invention is to provide a method of determining the relative amount of higher boiling constituents present in a distillate fraction to permit precise control of fractional distillation conditions. Another objective of this invention is to provide a means of determining the composition of process or product streams, for example, the composition of light hydrocarbon streams in alkylation, polymerization, and petrochemical manufacture. The method may also be used to control or monitor the composition of product streams, for example, liquefied petroleum gas commonly referred to as LPG and butane employed as a blending fraction for volatility control in gasoline.

This method and apparatus are useful in the determination of the composition of a volatile mixture, for exq ample, a mixture comprising a major portion of a normally gaseous component and a minor portion of a higher boiling component. Examples of such mixtures include propane containing a minor portion of butanes, butanes containing a minor portion of pentanes and isobutane containing a minor portion of normal butane.

Having set forth its general natu-re, the invention will be best understood from the more detailed description hereinafter which refers to the accompanying drawings.

FIGURE l shows an arrangement of apparatus in which the process of this invention may be practiced.

FIGURE 2 shows an assembly of the apparatus in a vented enclosure.

FIGURE 3 shows the relationship between the temperature measured in accordance with this invention and the percentage of pentane in a butane fraction.

FIGURE 4 shows the relation between the temperature ice determined by this method and the percentage of butane in a propane stream.

lFIGURE 5 illustrates the use of this method and apparatus in the control of a fractional distillation process.

FIGURE 1 illustrates the apparatus in one form which may 'be used in the process of this invention. It comprises a vertical tube 1 with an inlet 2 located near its mid-point. -Tube 1 is open at the top and filled with glass beads 3 retained in the tube by a glass wool plug 4 in the bottom. An outlet in the bottom of tube 1 is connected by a U-tube 5 with a T 6 open to the atmosphere through vent 7 and connected to a second vertical tube 10 through inlet 11. Inlet 11 is connected to tube 10 at about its midpoint. Like tube 1, tube 10 is open at the top and packed with glass beads retained by a glass wool plug at the bottom. A thermowell 12 is inserted in tube 10 at the-bottom and an outlet from the bottom of tube 10 is in communication with a U-tube 14 which is open to the atmosphere at a point above the level of thermowell 12. The apparatus may be fabricated of copper tubing, tubes 1 and 10 being about 3A inch and the inlets, T, U-tubes, and vent being 1%6 or 1A inch tubing. Other metals or glass are suitable and differences in dimensions may be used as convenient.

FIGURE 2 shows the apparatusof FIGURE l disposed in an air-tight vented box which may be used to prevent frost formation on the vaporization tubes when used for liquid streams boiling below 32 F. and to prevent variation in the rate of vaporization caused by ai-r currents or drafts. In FIGURE 2, line 20 is a connection to a sample source. Valve 21 is employed to close the sample source when the apparatus is not in use. The sample source is connected through valve 21 to a restriction tube 22. Restriction tube 22 is an elongated tube of small internal diameter -or capillary employed to provide a constant ow of liquid sample. Line 22 may be, for example, a tube of .018 inch inside diameter approximately 25 feet long. Tubing of smaller inside diameter -may be employed of correspondingly shorter length. Tube 22 is connected through the wall of air-tight metal cabinet 24 with inlet line 2 by means of union 23. Cabinet 24 is provided with a vent 25 and an observation window 26. Thermowell 12 extends through the wall of cabinet 24 and a thermometer or thermocouple is inserted in the thermowell 12.

In the method of this invention, normally gaseous liquid sample is introduced as a continuous stream at constant rate into a packed vaporizing-tube vented to the atmosphere. Upon introduction of the normally gaseous liquid into the first vaporizing tube, a portion evaporates cooling the remaining liquid by auto-refrigeration. Liquid flowing down over the packing is contacted in countercurrent flow with vapor generated by ambient heating of the descending liquid. This countercurrent contact in the first zone effects a stripping action so that any components lower boiling than the main body of normally gaseous liquidl are removed. This stripping prevents variation in quantity of low boiling constituents which may be in the liquid mixture from effecting the evaporation in the following vaporization zone. The stripped remaining liquid from the first zone is introduced into a second similar vaporizing zone where it is contacted in countercurrent Iilow with vapor again generated by ambient heating of the descending liquid. The residue liquid withdrawn from the bottom of the second contacting zone is at its boiling point at atmospheric pressure and the boiling point of this residue liquid corresponds to the proportion of higher boiling constituents in the originalY liquid mixture.

EXAMPLE A criterion ofthe amount of pentane in the distillate from a debutanizing distillation is determined by the method of this invention. The butane distillate is condensed, a portion is returned to the fractional distillation column as reflux and a portion is withdrawn as liquid product. A sample stream of the liquid product is passed through a capillary tube at constant rate into an initial vaporizing zone. The butane fraction is ilashed, vapors are Vented and the remaining cooled liquid is passed through a U-tube into a second vaporizing zone. Further vaporization is effected, the vapors are vented and residue liquid is collected in the bottom of the second vaporizing zone and withdrawn through a U-tube maintaining a liquid level 1/2 inch above the bottom. Liquid ows from the U-tube at a rate of 12 drops per minute. A thermocouple inserted-in the liquid at the bottom of the second vaporizing tube shows that thetemperature of the residue liquid is 34 F. A sample of the debutanizer distillate taken at the same time analyzed by laboratory low temperature fractional distillation shows it to contain 1.8 volume percent pentanes. The method of determining the boiling point of the liquid residue of this invention is applied to a liquid butane stream separated from a higher boiling hydrocarbon mixture containing isopentane. The relationship between the isopentane content of the butane distillate and the boiling point of the liquid residue obtained by this invention is shown in FIGURE 3.

In a process for isobutane-olefin alkylation, propane introduced in the feed streams is continuously removed by a depropanizing distillation of an isobutane stream. In this depropanizing distillation, propane is separated as distillate and isobutane as distillation bottoms. The relationship between the mol percent of isobutane in the liquid propane distillate and the boiling point of the liquid residue determined in accordance with this invention is shown in FIGURE 4.

The method of determining the boiling point of the liquid residue of a liquid mixture is-a simple method of continuously determining a criterion of the composition I of the liquid mixture. This criterion of composition may be used in the control of fractional distillation equipment to provide rapid control of fractionation conditions in response to changes in feed quality. Hcretofore analysis of the distillate products by laboratory low temperature fractional distillation has required expensive laboratory facilities and time consuming laboratory procedures and calculation. The time required in making such tests has made it impossible to control fractional distillation directly by the quality of the distillate so that control has necessarily relied upon indirect criteria 'such as the temperatures at some point in the distillation tower. Using the method of this invention the distillate may be continuously monitored and distillation conditions such as reboiler heat or retlux rate may be changed directly in response to the distillate composition to maintain a desired composition as shown in FIGURE 5.

In the application of this invention to the operation of a distillation tower, the optimum separation which can be obtained is established. The optimum separation for a given tower involves some overlapping, for example in depropanizing some loss of butanes overhead Without complete removal of propane from the buttoms. A cornpromise is selected between purity and recovery of the distillate or between retention of the higher boiling cornponent in or stripping of the lower boiling component from the distilland. Having established the degree of fractionation acceptable for a given '.distillation, this separation is expressed interms of the desired purity of the distillate and a corresponding boiling point of the liquid residue obtained by the method of this invention selected. The liquid distillate is then continuously monitored by the liquid residue boiling point method of this invention. If a change in feed stock quality or some other uncontrolled operating variable alters fractionation conditions the effect of this chan-ge upon distillate quality is immediately evidenced by a change in the boiling point of the liquid residue. If the boiling point of the liquid residue increases, this indicates that higher boiling material is increased in the distillate and the condition is corrected, for example, by reducing the reboiler temperature or the heat input. Conversely, a decrease in the boiling point of the liquid residue indicates incomplete bottoms stripping and is corrected, for example, by increasing reboiler temperature or heat input.

Referring to FIGURE 5, a distillation tower 41 is shown with feed line 40, overhead vapor line 42 and bottoms draw off line 43. Heat input is provided by passing a portion of the bottoms product through line 44 into contact with reboiler coil 4S and back to tower 41 through line 46. Overhead vapor in line 42 is condensed in condenser 60, a portion of the resulting liquid is returned to tower 41 as reflux through line 61 and liquid distillate product is discharged through line 62. A sample stream of liquid product is passed through sample line 20 and the assembly of apparatus illustrated in FIGURE 2 comprising shut oit valve 21, capillary expansion tube 22 and vaporization tubes 1 and 10. Thermocouple 29 measures the temperature of the residue liquid and by means of controller 50 and regulator 51 increases or decreases the heat input by adjusting the steam admitted to reboiler coil 45 through lines 52 and 53 and discharges through line 56.

Although this invention has been described in connection with the determination of the composition of hydrocarbon mixtures, the method is generally applicable to the determination of a criterion the composition of liquid mixtures containing a major portion of a normally gaseous constituent and a minor portion of `a higher boiling constituent. Although the temperature of the residue boiling point corresponds to the composition of the original liquid mixture, the temperature signal may be used directly in the control of a process variable to establish and maintain a desired liquid composition.

I claim:

1. A method of determining a criterion of the composition of a liquid mixture comprising a major portion of a normally gaseous component and a minor portion of a component higher boiling than said major component which comprises providing a feed 4to a distillation zone:

removing a vaporous overhead containing said major and minor components, condensing said overhead to provide said liquid mixture and using a part thereof as reux,

passing at least a portion of said liquid mixture through a capillary expander and thence into a first vaporization zone at atmospheric pressure vaporizing a portion of said mixture forming a cooled remaining liquid,

passing said remaining liquid to a second vaporization zone at atmospheric pressure vaporizing all but a minor residue of remaining liquid,

continuously withdrawing said minor residue from` said second vaporization zone,

and measuring the temperature of said minor residue at the point where withdrawn from said second vaporization zone.

2. The method of claim 1 wherein heat is supplied to said rst vaporization zone and said' second vaporization zone by ambient heatin-g.

3. The method of claim 1 wherein an operating condition of said distillation zone is controlled in response to said temperature whereby the proportion of said minor component is maintained at a selected level.

4. An apparatus comprising a distillation means,

means to introduce a vfeed into said distillation means,

means to remove vaporous overhead product from said distillation means,

means to condense said vaporous overhead forming a vaporizable liquid mixture and means returning a- 5 Y 6 a capillary eirpansion rneans to introduce a portion of References Cited by the Examiner iag gnzable liqu1d mixture 1nto a rst Vaponza- UNITED STATES PATENTS said first vaporization tube being open at its top, 2,299,899 10/1942 H oughland' means t0 convey liquid lfrom said first vaporization 5 2,475,957 7/1949 Gllflffe 62x37 tube to a second vaporization tube, 2,599,133 6/1952 Schllllllgsaid second vaporization tube being open at its top, 2,627,318 2/1953 SWefDdlOimeans to discharge liquid from said second vaporiza- 3,009,864 11/ 1961 Webb.

tion tube and, 3,034,307 5/ 1962 Berger 62-21 X means to measure the temperature of liquid discharged 10 from said second vaporization tube. NORMAN YUDKOFF, Primary Examiner. 

1. A METHOD OF DETERMINING A CRITERION OF THE COMPOSITION OF A LIQUID MIXTURE COMPRISING A MAJOR PORTION OF A NORMALLY GASEOUS COMPONENT AND A MINOR PORTION OF A COMPONENT HIGHER BOILING THAN SAID MAJOR COMPONENT WHICH COMPRISES PROVIDING A FEED TO A DISTILLATION ZONE; REMOVING A VAPOROUS OVERHEAD CONTAINING SAID MAJOR AND MINOR COMPONENTS, CONDENSING SAID OVERHEAD TO PROVIDE SAID LIQUID MIXTURE AND USING A PART THEREOF AS REFLUX, PASSING AT LEAST A PORTION OF SAID LIQUID MIXTURE THROUGH A CAPILLARY EXPANDER AND THENCE INTO A FIRST VAPORIZATION ZONE AT ATMOSPHERIC PRESSURE VAPORIZING A POR- 